(a) Field of the Invention
The present invention relates to a macro lens system, and more specifically to a macro lens system assuring aberrations favorably corrected within a wide magnification range from 1.7.times. to 8.times. and having a high aperture ratio of F/2.8.
(b) Description of the Prior Art
Generally speaking, a macro lens system should desirably permit varying magnification within a wide range so as to cover a wide magnification range with a single lens system. However, aberrations are remarkably varied by changing magnification levels can hardly be balanced adequately over the entire magnification range.
Further, in practical use of the macro lens system, it is brought into focus on the center of an image with the stop fully open and then the stop is closed until depth of field required for photographing is obtained. It is therefore necessary to preliminarily match the best central image position in fully open condition with that in the stop down condition. Moreover, the effective F number (Fe) which is expressed as Fe=F(1+.beta.) is larger and visual field of the viewfinder is darker as magnification .beta. is enhanced. In order to perform focusing with high precision, however, the macro lens system should desirably have brightness on the order of F/2.8.
As the conventional macro lens systems, there have hitherto been known those which were disclosed by Japanese published unexamined patent application No. 92230/79 and Japanese published examined patent application No. 3287/73. However, the former conventional example provides a macro lens system which is as bright as F/2.8 but is limited in its applicable range to magnification levels of 1.5.times. to 5.times.. The latter conventional example provides a macro lens system which has an aperture ratio of F/2.8 and a complicated composition comprising five components of seven lens elements, and another macro lens system which has relatively simple composition comprising four components of six lens elements but is not sufficient in brightness (F/4), thereby making focusing rather difficult.